It's definitely fake, the trailer showed a much more different sort of gameplay and the Pokémon's 3D models in the battle scene were cel-shaded than what was shown in that .gif animation.

That .gif image was probably made much more earlier in time, probably even before Black Version 2 and White Version 2 were made, probably just to show the speculated Pokémon Grey Version and demonstrating how it would've looked like if it was on the Nintendo 3DS. It's just like other fanmade images that surfaced from certain fan websites.

Either way, I much prefer the one from the trailer, it was much more nicer and the camera could turn around.

Edit: Wait! Hold on, the .gif image is actually from a YouTube video! Here it is:

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[Platinum Lucario's Signature]

Researching the NDS ROMs...

sure does feel like a brand new adventure!

I'm gradually learning to use Microsoft Visual Studio!

Always focus on the present, best thing to do! Never look back into the past and never worry about anything bad of the future!